1. Field of the Invention
The present invention relates to a synchronized code division multiplexing (CDM) communication method and a synchronized CDM communication system that implement communication between a central office (an optical line terminal (OLT)) and plural optical network units (ONU) using synchronized code division multiplexing.
2. Description of the Related Art
An optical access network system which is configured by connecting a central office and plural ONUs together with a PON (a passive optical network) is a representative example of a communication system between a central office and plural ONUs. The central office is equipment installed at a business (referred to as a center node, a service node or the like). The ONUs are equipment (referred to as edge nodes or the like) installed at subscribers (also referred to as users).
A PON is a network in which a optical splitter (also referred to as a star coupler) is connected partway along an optical fiber transmission line to split a single optical fiber transmission line into plural optical fiber transmission lines, and in which plural ONUs are connected in a star configuration around the optical splitter (see, for example, H. Onishi et al., “Ethernet PON Systems”, Fujikura Technical Review, No. 102 (April 2002), pp. 18-21 [Document 1], and Y Miyamori et al., “PON Based All Fiber-Optical Access System”, Hitachi Review, Vol. 47, No. 2 (1998), pp. 63-66). A PON is deployed in a network connecting between a central office and ONUs. Due thereto, the optical fiber transmission line between the central office and the optical splitter may be shared by plural ONUs. Therefore, equipment costs can be kept down.
Hereinafter, communication between a central office and N ONUs will be referred to as “1-to-N communication” (herein, N is an integer of at least two). In communication from the ONUs to the central office (hereinafter referred to as upstream communication), a synchronized CDM communication system that is configured using a PON has to multiplex encoded signals from plural ONUs at the same time. Therefore, communication timings of the plural ONUs must be synchronized. In general, the distance from the central office to the ONU is different for each ONU. Therefore, it is necessary to carry out ranging, which adjusts the communication timings, in accordance with the respective distances from the ONUs to the central office.
A related art ranging method is implemented as follows. First, the central office sends an echo transmission instruction to the plural ONUs. The ONUs receiving the instruction respectively return echoes to the central office. Then, from the echoes, the central office measures the respective distances to the ONUs that returned the echoes. Hence, the central office determines transmission timings in accordance with the respective distances to the ONUs, and synchronizes transmission timings of the plural ONUs (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2005-33544 and 2007-228134).
In synchronized CDM communication based on the above-described ranging method, when a new ONU, for which ranging has not been completed, is added to upstream communication, ranging must be performed for the newly added ONU too. Therefore, the central office must continuously carry on operations for discovering newly added ONUs. Accordingly, in addition to transmission data, it is necessary to reserve a time interval in transmission frames for discovering newly added ONUs (a discovery time interval).
Here, the meaning of the term “newly added ONU” includes a ONU that is added to a communication system when a user requests commencement of communication services. That is, an added ONU is a ONU that was not connected to the communication system at the start of operation of the communication system. The ONU is first connected to the communication system only after the user requests the commencement of communication services. For the discovery time interval, it is necessary to reserve a duration for an echo signal to travel both ways between the central office and a newly added ONU. Therefore, if such a time interval is provided in the transmission frame, a non-data transmission time interval in the transmission frame is extended, leading to a fall in transfer efficiency.